In a method of the mentioned type, the wavelength is adjusted by means of a wavelength selective filter element through which a beam of radiation is transmitted, whereby the adjusted wavelength depends on the angle of inclination of the filter element relative to the optical axis. A typical filter element of this kind is a Fabry-Perot etalon. The wavelength of the transmitted radiation is adjusted by tilting the etalon relative to the optical axis.
An example of an optical device wherein a Fabry-Perot etalon may be used as a wavelength selective filter element is a laser. A laser typically comprises a resonator cavity which is formed by two reflective elements, for example highly reflecting mirrors, and an active medium or gain medium arranged inside the cavity. The optical spectrum of the laser is limited by the spectral region over which the gain medium yields optical gain. For a semiconductor laser, for example, the typical width of the gain curve is 100 nm or more. Depending on the geometry of the cavity, the laser radiation has certain allowed spatial and spectral modes. The spatial modes of the laser are related to the light energy distribution transverse to the optical axis. The spectral modes which are also called longitudinal modes correspond to different wavelengths or frequencies of the light. The frequencies f.sub.N these modes are given by the formula: ##EQU1## wherein N is an integer, c is the speed of light and L is the optical path length of the laser cavity. Accordingly, the frequency spacing between two longitudinal modes thus is: ##EQU2## The allowed radiation spectrum in a laser cavity thus is a discrete distribution of modal frequencies limited by the gain curve. For many applications, the use of a laser output radiation with the described frequency spectrum, i.e., with a mixture of several frequencies, is satisfactory. There are, however, more and more applications, for example in optical communication technology, spectroscopy or holography, wherein operation of the laser at a single axial mode is required. In order to achieve such single mode operation, filter elements have to be inserted in the laser cavity which reduce the bandwidth of the radiation.
Common coarse filter elements are diffraction gratings, or birefringent filters, or acousto-optic or electro-optic elements. In many applications, however, such elements cannot ensure single mode operation of the laser.
A further bandwidth reduction of the laser radiation can be achieved by using additional filter elements. From EP-A-0268411, for example, it is known (see FIG. 1) to use a tuning plate of transparent material in an external cavity laser to adjust a narrow bandwidth output radiation. A preferred choice for narrow bandwidth (high finesse) filter elements are Fabry-Perot etalons. The use of such etalons as wavelength selective elements in lasers is known, for example from U.S. Pat. No. 4,947,398. Basically, there are two types of Fabry-Perot etalons:
a) The gas-spaced etalon comprises two elements of glass or similar material the parallel end faces of which are arranged opposite to each other. A wavelength tuning with such an etalon can be achieved by varying the distance between the parallel end faces using piezoelectric translators. An etalon of this type is known from U.S. Pat. No. 4,097,818.
b) The solid-type etalon has a much simpler design than the gas-spaced etalon. It consists of a plane-parallel plate of transparent material the front and rear surfaces of which are coated with a reflecting coating. Since such an etalon consists only of one piece, no alignment of the etalon itself is required. A wavelength tuning with such an etalon is performed by tilting the etalon relative to the optical axis. The wavelength .lambda. for which the etalon has maximum transmission is a function of the angle .alpha. of the surface normal of the etalon relative to the optical axis. The following relationship applies: ##EQU3## wherein n is the refractive index of the etalon material, d is the thickness of the etalon, and m is an integer characterizing the order of the transmission maximum.
The tilting is performed manually or by a motor-driven tilting arrangement. Typical tilting arrangements are tiltable or rotatable tables on which the etalon is mounted. The tilting axis in these arrangements is perpendicular to the optical axis. In order to achieve the required wavelength accuracy with such an arrangement, the mechanical precision of the drive for the etalon tilting and the positioning accuracy of the motor have to be very high. Such an arrangement is therefore complex and costly.